1. Field of the Invention
The invention relates generally to stationary position type optical readers and particularly to an object sensing system for a stationary position type optical reader.
2. Background of the Prior Art
Many available fixed position optical readers, typically termed "presentation scanners" are configured to operate in a continuous scan mode; that is they continuously capture and attempt to decode bar code symbols without regard to whether a symbol-bearing substrate is in the field of view.
A continuous scan operating configuration requires repetitive illumination flashing of an LED array in the case of an image sensor based optical reader and repetitive laser scanning in the case of a laser scan engine based optical reader. Repetitive flashing illumination or laser scanning requires a high level of energy consumption and can result in premature component degradation. Furthermore, the flashing illumination has been observed to be highly distracting to users of such optical readers configured to continuously scan image data.
U.S. Pat. No. 5,550,366 describes a system for automatically activating image scanning in a portable bar code reader wherein bar code image capturing and decoding is commenced when the presence of a bar code in a target area is detected. However, the detection of a bar code in the target area is carried out on a period basis and requires activation of a high radiance source of illumination. Accordingly, the system is not responsive in real time to an object being moved into the field of view of the reader, and the high radiance illumination required for operation of the system remains a source of distraction.
There is a need for a fixed position optical reader which automatically reads indicia on objects presented to the reader without continuously scanning images when objects are not present in the field of view of the reader's imaging assembly.